1. Field of the Invention
The invention pertains to an electro-static image sensor which gives an electrical signal that represents an image carried by non-visible radiation, for example, X-rays. In this type of image sensor a latent image is recorded, in the form of a spatial distribution of space charge, at the interface between a layer of photoconductive material and a layer of dielectric material. Then, this latent image is read point by point by compensating this space charge with photocarriers released by a pencil of light scanning the layer of photoconductive material. Two electrodes are used to receive an electrical signal. Storage at the interface between the photoconductive layer and the dielectric layer enables an image to be analyzed and processed at any pace, irrespective of the time for which the sensor is exposed to radiation.
2. Description of the Prior Art
In the prior art, the American U.S. Pat. No. 4,446,365, for example, describes the making of an image sensor comprising a layer of dielectric comprising a sheet of polyethylene terephtalate (PETP marketed as Mylar by DUPONT DE NEMOURS), a layer of amorphous selenium and two electrodes between which the dielectric layer and the selenium layer are located. During the recording of a latent image, a voltage of 2000 volts is applied to the electrodes in order to polarize the layer of photoconductive material. During the stage for reading the latent image, the two electrodes are connected to the input of a current amplifier which converts the intensity of the discharging current of the layer of photoconductive material, under the effect of light scanning, into a video signal which can be observed on a cathode tube.
This type of image sensor can be used, in particular, instead of photographic films in conventional installations for radiological examination. In an application of this type, the sensor is placed in a conventional installation, in the place of the photographic film, to record a latent image. Then, it is placed in a reading device to restore a picture in a cathode tube. The latent image persists for a few hours: thus the picture can be read and recorded at two different places if need be.
The image sensor of the known type has the disadvantage of requiring a high voltage source during the recording stage. This implies a modification of conventional radiology installations to incorporate the high voltage source into them.
An object of the invention is to enable a latent image to be recorded in the sensor without a voltage source being needed for it.
An object of the invention is an electrostatic image sensor with the same structure as the device of the known type, but one where the layer of dielectric material comprises an electret designed to polarize the layer of photoconductive material electrostatically.